The present invention relates to the improvement of a fixing apparatus in an image forming apparatus for forming an image according to electrophotographic method.
A heat roller type fixing apparatus using a pair of rollers is a fixing apparatus to be commonly used. To meet the requirements for color images and diversified recording media, a fixing apparatus using a belt is proposed. The fixing apparatus using a belt has the following advantages. The belt driven along the roller makes it possible to increase the width of the nip gripping a recording medium, and to prolong toner heating time, thereby ensuring improved heating efficiency. Thus, fixing by use of a belt is effective when there is a thick toner layer forming an image as in the case of a color image or when the transparency of an image is increased by raising the level of toner fusing, as in the case of an overhead transparency film (OHP film).
Belt type fixing apparatus are disclosed in the Official Gazette of Japanese Application Patent Laid-Open Publication Nos. Hei 05-150679 and 2000-172103, for example.
As described in the aforementioned Official Gazettes of the Japanese Patent, the belt type fixing apparatus is basically characterized in that a fixing roller forming a nip is brought into pressure contact with a belt which is supported by supporting rollers. The heat source for heating a toner image is provided inside the support roller or the fixing roller in the fixing unit.
This configuration inevitably causes a convex nip to be formed on one side by the fixing roller. A recording medium passing through the convex nip on one side is often deformed and curled. Since recording medium is heated at the nip, it is deprived of water and the shape of the nip tends to make the recording medium to curl. This raises problems in the conveyance of the recording medium after fixing process, with the result that recording media are ejected irregularly. Further, a sufficient nip width cannot be ensured to reduce the curl, so that a sufficient fixing performance cannot be obtained, according to the prior art.